In the first eighteen months of this three year approved program, we have demonstrated that respiratory muscle fatigue is associated with a spectral shift to lower frequencies in EMGs recorded from the human diaphragm and sternocleidomastoid muscles, whereas the change cannot be detected reliably in exteral intercostal signals because of frequent interference from the more superficial pectoralis major. During the remaining time we shall address the following questions that remain unanswered: 1. What is the temporal relationship between the electrophysiologic and mechanical manifestations of fatigue? 2. What factors contribute to, or retard the onset of, the condition; and specifically what are the effects of hypoxia, acidosis and hyperinflation? 3. What is the prevalence of muscle fatigue in patients with respiratory disease? To study these problems we shall utilize the techniques for power spectral density analysis already developed in the laboratory, and apply them to signals recorded in normal human subjects and from patients in respiratory failure, prior to the institution of ventilatory support and during the weaning process. The role of factors such as hypoxia will be characterized further in an animal model (cat diaphragm) that preliminary studies suggest resembles the human muscle. The ultimate purpose of the research is to develop a rapid-response monitoring system for clinical use.